1. Field of the Invention
The present invention relates to voltage input circuits, and in particular, to low power voltage input circuits with high noise immunity and fast operating times.
2. Description of the Related Art
Input circuits such as voltage input circuits may be configured to monitor an input voltage and provide an output to an external, electrically-isolated circuit when the input voltage changes by a given threshold. Input circuits may be used, for example, to monitor whether circuit breakers are in the closed or open state. For example, an auxiliary contact of a circuit breaker may be used to make or break a connection of a station battery which is typically 125 V DC. When the breaker is closed, the input auxiliary contact of the breaker closes, which connects the battery voltage to the input circuit so the input circuit may monitor whether the breaker is closed or open.
It is thus necessary to monitor the voltage delivered to the input circuit so that it can be known whether the breaker is open or closed. An input switch connected in parallel across the input terminals of the breaker will receive close to zero voltage when the breaker is open, and will receive the battery voltage when the breaker is closed and the battery is thus connected across the input terminals of the input circuit. Input circuits are typically designed to generate an output signal of one state when voltage below a predetermined threshold is applied to the input of the input circuit, and-to generate an output signal of a second state when the input voltage is above a predetermined threshold. Input circuits typically generate a signal or produce an output that may be detected by an electrically isolated detection circuit so that, among other reasons, common mode noise is not transmitted from the bus and digital input circuit to the detection circuit.
Referring now to FIG. 1, there is shown a circuit diagram of a prior art input circuit 100. As will be understood by those skilled in the art, when voltage V rises above a certain threshold level, current I will flow through light-emitting diode ("LED") 110 where it may be detected by an external, electrically isolated light-detector circuit 111. Thus, whenever a voltage V applied to circuit 100 changes from a below-threshold level to an above-threshold level, detector circuit 111 can detect the occurrence of this change. This will occur when, for example, the contact connecting the battery to the input terminals of circuit 100 is closed, thereby changing voltage V from roughly zero volts to approximately 125 volts. As is known to those skilled in the art, LED 110 in combination with the electrically isolated circuit 111 designed to detect the light emitted by diode 110 may be part of an integrated opto-coupler device 112.
When such circuits are utilized for digital bus protection, a large number of such circuits may be required. Because a large number of devices can draw a large amount of power, there is a need for digital input circuits having low power requirements. Additionally, it is important that a digital input circuit maintain a fast response (or operating) time despite the low power requirement. It is also important that the digital input circuit have a high immunity to spurious noise such as transients or spikes, despite the need for fast operating time and low operating power, so that it does not erroneously indicate that an "on" input has been received when the input is actually "off", or erroneously indicate that the input has changed to the "off" state when it actually remains in the "on" state.
It is accordingly an object of this invention to overcome the disadvantages and drawbacks of the known art and to provide an input circuit having low power requirements.
It is a further object of this invention to provide such an input circuit that has a fast response time and high noise immunity.
Further objects and advantages of this invention will become apparent from the detailed description of a preferred embodiment which follows.